The present invention relates to a method and system for stable compliance control at a high speed by cooperation between a manipulator and a wrist body which is located at the end of the manipulator arm and is capable of high speed compliance control.
In many instances the work involved using a manipulator is restricted by the external environment. Parts assembly, crank rotation and grinding work are examples of operations in which an external environment affects a manipulator. In order to perform these operations using a feedback type manipulator based on position control, a high-precision position determining capability and an accurate teaching technique are necessary. However, the high-precision determining capability greatly increases the cost and it is difficult to obtain sufficient precision using present techniques. The exact work orbit teaching also requires much time. On the other hand, it has been proposed that the manipulator be provided with the flexibility to adapt to the external environment restriction by controlling the interactive force according to the restrictive environment. The control of the flexible operation adapting to such environmental restriction is called "compliance control" and the operation realized thereby is called "compliance operation". In other words, the compliance control controls the manipulator hand as if there were a suspension mechanism comprising a spring (Ks), a damper (Kd), a mass (M) and a source of force (fr) upon the hand as shown in FIG. 2.
As is disclosed in a paper entitled "Historical Perspective and State of the Art in Robotic Force Control", by D. E. Whitney, Proceedings of IEEE International Conference on Robotics and Automation, pp. 262-268, 1985 and the others, most of the conventionally reported compliance control methods are ones which feed back the force information from the external environment using a force sensor, or ones in which the whole arm is composed of an actuator capable of simple torque control such as a direct drive motor.
Although the direct drive arm using a direct drive motor can easily control the compliance, the manipulator itself is apt to be large relative to the moving weight due to the problem of the power weight ratio of present direct drive motors. Furthermore, it becomes a problem from the viewpoint of control that the response frequency decreases remarkably with an increase of compliance due to the inertial effect of a large manipulator weight.
The use of the manipulator which is provided with a decelerator and represented by an industrial manipulator brings about a larger power to weight ratio, making it possible to lighten the manipulator itself. However, since the influence of the deceleration friction becomes large, it becomes difficult to control the force of each joint with high accuracy. A method of obtaining compliance control by mounting the force sensor on the manipulator provided with a decelerator is disclosed in a paper entitled "Implementing Compliance Control with a High Speed Operator" by Hiroshi Ishikawa, et al., Japanese Precision Engineering Institute, Proceedings of the Spring Meeting, 1988, pp. 675-676. Its contents are also disclosed in the specification of Japanese Patent Application 63-59574. According to the disclosed method, the position and the velocity which the hand is to have at present are calculated from the force information obtained by the force sensor, and the position and velocity servo control is executed for each joint. Nevertheless, according to this method, the influence of the external environmental contacted by the end effector is included in the feedback loop, therefore, the system stability is affected by the external environment. Especially when the external environment is very hard, the apparent feedback gain becomes large and the control gain of the manipulator must to be reduced. However, the reduction of the control gain causes the decelerator friction to have a greater affect, thereby greatly reducing the force accuracy. Hence, the realizable area of the compliance is restricted by the decelerator friction and the hardness of the external environment.
In the meantime, a method was disclosed in a paper entitled "A Six Degree-of-Freedom Magnetically Levitated Variable Compliance Fine Motion Wrist" presented by R. L. Hollis, et al. at 4th International Symposium on Robotics Research, Santa Cruz, Ca., Aug. 9-14, 1987, which discloses a wrist body capable of high-precision compliance control on the manipulator, and after the wrist body is moved by the manipulator to the position where the work is to be done and the manipulator is fixed in position, the compliance control is activated by the wrist body. Yet, there arises a problem that a large movable area cannot be obtained during the compliance control since the manipulator position is fixed during compliance control by the described method.
For complex operation with the manipulator, such as assembly work, the capability of realizing arbitrary or adjustable compliance of the hand and changing the compliance freely is required. The present invention describes a method capable of stable compliance operation over a wide range of motion, which responds at a high speed to an extraordinarily hard object while contacting it very softly, in a manner impossible with conventional compliance control.
According to the present invention it becomes possible to touch a very hard object to be worked, making it possible to perform assembly and fitting between hard parts made of metal and the like. Such assembly of hard parts is highly desired at factory sites. Conventionally, such work may be performed only by an expensive direct drive arm. Moreover, according to the present invention, it now becomes possible to perform a high speed compliance operation which is not possible with the direct drive arm. The high speed compliance operation has also been in great demand at factory sites.
In order to realize a small inertia compliance operation which is not influenced by the external environment, the end of the manipulator is provided with a wrist body capable of compliance control. As described above, this structure has the problem that a large range of motion cannot be obtained during compliance control. The present invention enlarges the range of motion of the wrist body by moving the wrist body with the manipulator to a position where the wrist body can be effectively operated at all times.
Wrist bodies driven by a voice coil motor or a periplanar coil motor are employed. By using these actuators, a relatively linear force-current characteristic may be obtained, therefore, compliance control becomes possible without force feedback. Namely, since the force from the external environment does not enter into the feedback loop, stable compliance control may be carried out without any influence from the external environment. Furthermore, high speed motion becomes possible because of the small inertia of the wrist body. In order to cope with the small range of actuator motion, and the manipulator moves the wrist body to a position where the wrist body actuator can operate effectively at all times. A description of the wrist body structure of six degree-of-freedom motion and the control algorithm, for a manipulator and wrist body having six degree-of-freedom, will be described hereinafter.
A suitable example of the six degree-of-freedom wrist body is disclosed in the above paper by R. L. Hollis et al. and its contents are also described in the specification of Japanese Patent Application No. 63-131799. The following cites a part of the contents of the above patent application.